Most modern transceivers have a built in interface. These interfaces work well
in digital modes, but seem to lack support in analog voice modes. A case in
point is the Yaesu transceivers that only permit the use of their internal
interface when the transmitters are set to “Digital Mode”.

Commercial external interfaces cost 100 Euro or more, depending on the
additional futures. Due to the fact that most ham operators are on a limited
budget, a homebrew approach is attractive. After examining various ways to
interface the transceiver, it was found that the most practical way was using an
Optocoupler via a COM port.

The gain of using an Optocoupler in this project is obvious. First of all, the
total number of components is reduced, and the electric isolation between the
transceiver and the computer is achieved. Last but not least, the total cost of
the interface is less than 5 Euro --quite a difference compared to the prices of
the commercial interfaces available.
The voltage swing on
these lines is
-12v/-5v through +12v/+5v, so a Optocoupler can handle quite nicely the
negotiation between the rig and the computer.

The
interface is needed so the computer can send a request to the transmitter to
transmit or receive. The most practical method is via a serial COM port using
the RTS or DTR lines. Through these lines the PTT (Push to Talk) on the
transmitter is activated. The analog audio is passed from the sound card
directly to the audio in of the transceiver by simple cable connection.

DESCRIPTION OF SIMPLE INTERFACE CIRCUIT

The circuit described below was made to connect the computer to the transmitter.
It was designed as simple as possible to keep the number of components as low as
possible.

As soon as the program activates the RTS line (or the DTR) the negative voltage
of the line turns to positive. This makes the Led D1 as well as the Led in the
Optocoupler U1 to light, making the emitter and the collector of the Optocoupler
to conduct, thus giving the ground needed to the transmitter to TX.

The Led D1 is not necessary for the circuit to work but it is a very useful
visual indication of the status of the interface. An upgrade to this simple
interface was to replace the Led with a dual polarity Led. This way, instead of
having the led light up on TX, the led is green on Stand by (RX) and turns Red
on
TX.

DESCRIPTION
OF AN ADVANCED INTERFACE CIRCUIT VERSION for Myhna

In most cases the simple interface will be adequate for typical amateur use. In
our case it is not enough. The reason is quite simple: Myhna uses standard
prerecorded messages for typical transmissions. What happens if it is required
to send information that is not included in the standard messages?

A microphone override circuit is necessary. As soon the microphone is activated
all audio from the computer is disabled and is shifted to the microphone for
necessary manual operations. When the microphone PTT in deactivated the
interface returns control to the program.